@realcaseyrollins Its registration was open to everyone until recently. (However, it doesn’t comply with GDPR and UK GDPR requirements.) It changed to invitation-only after the number of users exceeded its capacity.
Misskey platform has "channels," which is their unique system to accumulate posts on similar topics. My previous post includes the link to Blobcats channel. The posts in channels aren’t open to other Fediverse servers due to the system limitation.